core-js/src/es6_symbol.js

Standard library

// ECMAScript 6 symbols shim
!function(TAG, SymbolRegistry, setter){
  // 19.4.1.1 Symbol([description])
  if(!isNative(Symbol)){
    Symbol = function(description){
      assert(!(this instanceof Symbol), SYMBOL + ' is not a ' + CONSTRUCTOR);
      var tag = uid(description);
      setter && defineProperty(ObjectProto, tag, {
        configurable: true,
        set: function(value){
          hidden(this, tag, value);
        }
      });
      return set(create(Symbol[PROTOTYPE]), TAG, tag);
    }
    hidden(Symbol[PROTOTYPE], TO_STRING, function(){
      return this[TAG];
    });
  }
  $define(GLOBAL + WRAP, {Symbol: Symbol});
  
  var symbolStatics = {
    // 19.4.2.1 Symbol.for(key)
    'for': function(key){
      return has(SymbolRegistry, key += '')
        ? SymbolRegistry[key]
        : SymbolRegistry[key] = Symbol(key);
    },
    // 19.4.2.4 Symbol.iterator
    iterator: SYMBOL_ITERATOR,
    // 19.4.2.5 Symbol.keyFor(sym)
    keyFor: part.call(keyOf, SymbolRegistry),
    // 19.4.2.13 Symbol.toStringTag
    toStringTag: SYMBOL_TAG = getWellKnownSymbol(TO_STRING_TAG, true),
    pure: safeSymbol,
    set: set,
    useSetter: function(){setter = true},
    useSimple: function(){setter = false}
  };
  // 19.4.2.2 Symbol.hasInstance
  // 19.4.2.3 Symbol.isConcatSpreadable
  // 19.4.2.6 Symbol.match
  // 19.4.2.8 Symbol.replace
  // 19.4.2.9 Symbol.search
  // 19.4.2.10 Symbol.species
  // 19.4.2.11 Symbol.split
  // 19.4.2.12 Symbol.toPrimitive
  // 19.4.2.14 Symbol.unscopables
  forEach.call(array('hasInstance,isConcatSpreadable,match,replace,search,' +
    'species,split,toPrimitive,unscopables'), function(it){
      symbolStatics[it] = getWellKnownSymbol(it);
    }
  );
  $define(STATIC, SYMBOL, symbolStatics);
  
  setToStringTag(Symbol, SYMBOL);
  
  // 26.1.11 Reflect.ownKeys (target)
  $define(GLOBAL, {Reflect: {ownKeys: ownKeys}});
}(safeSymbol('tag'), {}, true);